What is the mechanism of action of oral phosphodiesterase (PDE) inhibitors in the lungs?

From the Guidelines

Oral phosphodiesterase (PDE) inhibitors work in the lungs by blocking specific enzymes that break down cyclic nucleotides, primarily cAMP and cGMP, which are important second messengers in cellular signaling, as shown in the 2016 European Society of Cardiology (ESC) and European Respiratory Society (ERS) guidelines 1.

Mechanism of Action

When these inhibitors are taken orally, they enter the bloodstream and reach the pulmonary vasculature and airways, where they prevent the degradation of these cyclic nucleotides. This leads to increased intracellular levels of cAMP or cGMP, depending on which PDE isoenzyme is targeted.

• For example, PDE5 inhibitors like sildenafil (Revatio, typically 20mg three times daily) and tadalafil (Adcirca, usually 40mg once daily) specifically target PDE5, which breaks down cGMP.
• The elevated cGMP levels cause relaxation of vascular smooth muscle in the pulmonary arteries, resulting in vasodilation and reduced pulmonary vascular resistance, as noted in the 2009 guidelines for the diagnosis and treatment of pulmonary hypertension 1.

Clinical Applications

This mechanism makes these medications effective for treating pulmonary arterial hypertension.

• Other PDE inhibitors like roflumilast (Daliresp, 500mcg once daily), a PDE4 inhibitor, increase cAMP levels in inflammatory and immune cells in the lungs, reducing inflammation and bronchoconstriction, which is beneficial in chronic obstructive pulmonary disease.
• The specificity of different PDE inhibitors for particular PDE isoenzymes allows for targeted effects in the lungs while minimizing systemic side effects, though patients may still experience headaches, flushing, or gastrointestinal disturbances depending on the specific medication, as reported in the 2007 medical therapy for pulmonary arterial hypertension guidelines 1.

Key Points

• The most recent and highest quality study, the 2016 ESC and ERS guidelines 1, supports the use of oral phosphodiesterase inhibitors in the treatment of pulmonary arterial hypertension.
• Sildenafil, a PDE5 inhibitor, has been shown to improve exercise capacity, symptoms, and haemodynamics in patients with pulmonary arterial hypertension, with a recommended dose of 20mg three times daily 1.

From the FDA Drug Label

Sildenafil is an inhibitor of cGMP specific phosphodiesterase type-5 (PDE-5) in the smooth muscle of the pulmonary vasculature, where PDE-5 is responsible for degradation of cGMP. Sildenafil, therefore, increases cGMP within pulmonary vascular smooth muscle cells resulting in relaxation In patients with PAH, this can lead to vasodilation of the pulmonary vascular bed and, to a lesser degree, vasodilatation in the systemic circulation.

The mechanism of action of oral phosphodiesterase type 5 (PDE-5) inhibition in the lungs involves the inhibition of PDE-5 in the smooth muscle of the pulmonary vasculature, leading to an increase in cGMP and subsequent relaxation of pulmonary vascular smooth muscle cells. This results in vasodilation of the pulmonary vascular bed, which can be beneficial in patients with pulmonary arterial hypertension (PAH) 2.

• The selectivity of sildenafil for PDE-5 is important, as it is approximately 4,000-fold more selective for PDE-5 than for PDE-3, which is involved in the control of cardiac contractility.
• The inhibition of PDE-5 in the lungs can lead to a decrease in pulmonary vascular resistance and an increase in oxygenation.
• The clinical benefits of PDE-5 inhibition in the lungs include improved exercise tolerance and reduced symptoms of PAH 2.

From the Research

Mechanism of Action of Oral Phosphodiesterase Inhibition in the Lungs

• The mechanism of action of oral phosphodiesterase inhibition in the lungs involves the inhibition of phosphodiesterase type 5 (PDE-5), an enzyme that metabolizes cyclic guanosine monophosphate (cGMP) 3.
• cGMP is a key mediator of the relaxation and growth inhibition of vascular smooth-muscle cells, including those in the lung 3.
• By inhibiting PDE-5, oral phosphodiesterase inhibitors such as sildenafil and tadalafil increase the levels of cGMP, leading to the relaxation of pulmonary vascular smooth muscle and a decrease in pulmonary artery pressure 3, 4.
• This mechanism of action is thought to be responsible for the improvement in exercise capacity, WHO functional class, and hemodynamics seen in patients with pulmonary arterial hypertension (PAH) treated with oral phosphodiesterase inhibitors 3, 4.

Effects on Pulmonary Arterial Hypertension

• Oral phosphodiesterase inhibitors have been shown to be effective in improving exercise capacity, WHO functional class, and hemodynamics in patients with PAH 3, 4.
• These agents have also been shown to reduce the incidence of clinical worsening and improve health-related quality of life (HR-QOL) scores in patients with PAH 4.
• The use of oral phosphodiesterase inhibitors in combination with other therapies, such as endothelin receptor antagonists, has been shown to be associated with improved survival and reduced morbidity in patients with PAH 5.

Pharmacology and Therapeutics

• Oral phosphodiesterase inhibitors such as sildenafil and tadalafil are well-tolerated and have a favorable safety profile 4, 3.
• These agents are available in oral formulations and are typically administered once or twice daily 4, 3.
• The choice of oral phosphodiesterase inhibitor and dosing regimen may depend on individual patient factors, such as the severity of PAH and the presence of comorbidities 5.